Performance: Remove Hierarchical Select's form cache and don't reconstruct the entire form anymore

This will strongly affect hierarchical_select_after_build() and _hierarchical_select_submit().

And it will remove the need to run _hierarchical_select_form_has_hierarchical_select(), which severely affected performance in some situations: #376159: _hierarchical_select_form_has_hierarchical_select() is expensive (and called from hook_form_alter()).

I'm not entirely sure anymore why I chose to reconstruct the entire form before rendering just the necessary HS form item. This is what makes HS slow on some node forms, because the node form itself is slow to reconstruct. Removing the need to reconstruct the entire form can speed things up *tremendously*.

This is a performance feature.

And will not be for HS 3.0 for D6.

For D6. More like 3.5. 4.0 will involve API changes.

3.1, 3.2: minor bugfix releases. This is just an optimization and will not change anything visibly.

Interested in writing this? Basically it will require either POSTing all required data or passing most or all of it via a GET request (i.e. via the Drupal path). Not sure if that will be sufficient though. We also have to keep the security implications in mind. If you want to do it, I'll outline what exactly you have to do. Should not be too hard.

Tagging for HS4. Included in the HS4 roadmap: http://drupal.org/node/1052670.